The present invention is an improved position-measuring transformer of the type comprising precision scale elements used for extremely accurate measurement of linear or angular displacements. The noncontacting, inductively or capacitively coupled elements can be directly attached to fixed and moveable members of machine tools, navigational systems, fine control systems, and other precision mechanisms. Electrical output signals can drive readout displays, generate computer input data, and provide servo feedback signals.
The elements of a linear transformer are precision printed circuit patterns with parallel hairpin turns (windings), i.e., series-connected hairpin like conductors arranged in alternate north and south poles, repeated along the adjacent faces of two parallel flat bars. In the case of a rotary configuration transformer, these elements comprise series-connected radially disposed hairpin conductors arranged on the adjacent faces of two coaxially supported disks. These elements comprise primary and secondary windings. In these known air-core devices the spacing of the conductors is the same on both elements, and when the series-connected conductors of the primary winding are energized with an alternating current, the current in the conductor of that winding induces a current in the conductor of the secondary winding which is adjacent thereto. These voltages at the various windings add together to give a secondary voltage which varies in magnitude according to the relative position of the conductors of facing primary and seconary windings. The induced (secondary) voltage is at a maximum when the poles of the conductors face each other. As one element (the rotor) moves, the induced voltage passes through zero and then rises to a negative maximum upon the next incidence of poles confronting each other. Thus, the secondary voltage induced by the primary current is a function of the relative position of the transformer elements. This function is termed the coupling wave.
In general, the induced output voltage will not be a pure sine or cosine function, but it will be a periodic function with a period equal to double the input conductor or pole spacing. It may be considered to be the sum of a sine (or cosine) curve plus a series of harmonics.
Two-phase operation is achieved by providing two independent windings on the transformer stator with 90.degree. phase difference (with respect to each other) in space phase (not time phase). One set of windings is displaced in one-quarter space cycle from the other; the windings on the stator are arranged in groups to permit this displacement. The resulting coupling waves provide paired voltage values which are unique for each position within a full space cycle.
The principle of the linear position-measuring transformer is exactly the same as that of the rotary transformer, linear distances being equivalent to angles. The stator of the linear transformer is known as the slider, and the rotor as the scale. Either slider or scale may comprise the moving element, the other being stationary.
In the rotary transformer, the induced signals are averaged over the entire circular pattern, in the linear transformer, the signals are averaged over a distance corresponding to a substantail number of cycles usually 32 or 48 cycles, but not restricted to any particular number of cycles.
In the rotary transformer according to the prior art, there may be one or more primary windings and one or more secondary windings, all primary windings (if plural) being fixed with respect to each other and all secondary windings (if plural) being fixed with respect to each other, and all primary windings being movable as a unit with respect to all secondary windings. Either winding or group of windings so fixed with respect to all other windings may serve as the primary or as the secondary winding or windings of the transformer, and the terms "rotor" and "stator", if desired, may be interchangeably applied to either of such groups of relatively fixed windings and to the support therefor. Hereinafter the term "member" will be applied to any one of such groups of windings of a transformer, together with the support or supports therefor. Conveniently, however, the member including a continuous winding may be referred to as the rotor or primary member of the transformer.
A transformer useful in measuring the relative angular position of two shafts may be produced by combining two members, for example, a rotor and a stator, confronting each other.
A typical rotor according to the prior art carries a single multipolar winding extending over 360.degree. of a circle. A disk of any suitale material upon which a conductive pattern is produced usually by photoeteching a copper layer bonded to a disk material with an insulating adhesive layer, which is preferably substantially planar, carries a single multipolar winding. The winding includes a multiplicity of radially extending strip-like conductors connected in series by circumferential conductors in series by circumferential conductors so that alternate conductors carry current in the same direction, whereas adjacent conductors carry current in radially opposite directions. The conductors are identical in shape and are spaced at uniform angular intervals in a circular arcuate array about the center of a pattern which they establish, the center becoming the effective center of the disk. These conductors cover in uniform fashion the 360.degree. of angle about the disk center.
In the case of both the stator and the rotor, there is one pole per radial conductor, each such conductor forming a pole, and the pole spacing, for example, being about 1.degree.. Transformers according to the prior art are not restricted to any particular number of conductor groups or poles. The number must be even, however, in order that, for a given polarity of energizing voltage applied to the winding, the sense of the magentic fields appearing adjacent the disk face in front of the individual radial conductors will alternate all the way around the disk, and in particular in the two adjacent conductors at which the winding terminates.
The conductors are laid down in the form of a metallic pattern, for example, one made of copper, by a photoetching process, and must be positioned with a relatively high degree of accuracy, although the large number of conductors provided effects an averaging process in the overall coupling between the two transformer members whereby the effect of deviations of individual conductors from their proper positions is decreased or reduced. Transformer members according to the prior art have inner and outer radii for the conductor patterns of the order of 1/2 and 11/2 inches, respectively, although such transformers are not limited by any particular dimensions.
Assuming transformer members each having N poles corresponding to N radial conductors, there will be N relative angular positions of the two members in which each conductor of one member is parallel to and at a minimum separation from one conductor of the other member. N/2 of these positions represent positions of maximum coupling of one sign between the transformer members, whereas the other N/2 positions represent positions of maximum coupling of the opposite sign. The coupling wave goes through N/2 cycles for one relative revolution of the two transformer members. The coupling function or coupling wave of the complete transformer is the sum of the contributions of each of the series-connected secondary winding conductors.
According to the prior art, the conductors are dimensioned to minimize harmonic components in the coupling wave between members having uniformly and equally spaced conductors. The preferred spacing and width of the conductors is discussed in the prior art. Reference is made to U.S. Pat. No. 2,799,835 of Tripp et al.
According to another feature of the prior art, the coupling due to current flow in the circumferential conductors is suppressed by dividing the winding of one of the transformer members (e.g., the stator) into a plurality of sectors so that in one or more sectors of that member the circumferential flow of current is clockwise, while in one or more other sectors the circumferential flow of currrent is counterclockwise, the sectors of clockwise flow subtending at the disk center the same angle as do the sectors in which the circumferential current flow is counterclockwise.
The stator according to the prior art comprises a disk which may be similar to the rotor disk. Laid down on the disk surface are a plurality of conductor groups or sectors, each including a plurality of series-connected radial conductors. Harmonic compensation according to the principles of the above-noted patent may be incorporated into members of this type.
A typical transducer comprises a number of conductor groups, equiangularly spaced from each other. Each group comprises series-connected radial conductors. In use, alternate groups are connected together in series by means of external leads to produce two windings, each of which links one half of the conductor groups or sectors in space quadrature of the pole cycle rotor. The interconnection of the conductor groups or sectors by leads is such that in each of the resulting windings the direction of circumferential current flow is reversed in successive conductor groups of that winding.
Rotary position-measuring transformer may be used in three different ways: (1) in pairs, as angular data transmitter and receiver, in a similar manner to the use of synchros and resolvers; (2) as a device for absolute angular measurement; or (3) for producing an angle in response to a control input.
According to the prior art, a rotary position-measuring transformer may be used as an angular data transmitter, by applying a single-phase AC voltage to the rotor windings. The voltage output from the two-phase stator windings will then be proportional to the sine and cosine of the angular position of the rotor with respect to the stator and this output voltage may be compared to the output voltage from a receiver transformer mechanically coupled to the device whose position is to be controlled.
The receiver transformer has its stator windings excited by the corresponding transmitter stator windings. The output of the receiver rotor is the position error signal, and is zero when the transmitter and receiver are at complementary angular positions. Since there are N nulls per revolution, where N is the number of poles, the transformer is electrically a multiple-speed device. Mechanically, however, it is a one-speed device. To avoid ambiguity, in the prior art it was necessary to use a two-speed servo system, and incorporate another component to give a one-speed electrical signal. The accuracy of this one-speed "coarse" data device need only be sufficiently good to assure that the switchover to "fine" operation with the transformer occurs in the general vicinity of the required null, or within about one-quarter of a transformer cycle. The devices of the prior art have used conventional synchros or resolvers for this purpose, and provision for a one-speed data device is made in the standard rotary transformer assembly. Another method by which "coarse" data has been provided according to the prior has been to provide a separate one-speed transformer pattern on the same set of disks but this method requires larger disks.
The same three basic methods of use which are characteristic of the rotary position-measuring transformer also apply to the linear position-measuring transformer; the only difference between the two devices is that the angle of the rotary form becomes a linear distance in the linear form. For example, the linear transformer may be used in transmitter-receiver applications in the same way as the rotary transformer, to accurately reproduce linear movements at a distant point. Again, a "coarse" control is necessary to avoid ambiguity. The usual method of providing this course control is to use a rotary synchro or resolver operated through a rack and pinion or a lead screw. The accuracy of this coarse data element need be sufficient only to assure that the switchover to "fine" operation with the transformer occurs in the vicinity of the null, or about one-quarter of a transformer cycle, i.e., 0.025 inch along the linear scale.